1. Field of the Invention
The present invention is related to an organic electroluminescence (light emitting) element and a display having the element. In particular, the present invention is related to an organic electroluminescence element having an organic light emitting medium layer of a layered product structure comprised of a polymer compound and an inorganic compound and a display having the element.
2. Description of the Related Art
In organic electroluminescence (organic EL) elements, a voltage is applied to an electrically conductive organic light emitting medium layer, and thus electrons injected from a cathode and holes from an anode are allowed to recombine, whereby an organic light emitting material which makes up the organic light emitting layer is allowed to emit light upon this recombination. (Here, an organic light emitting layer with a hole transport layer, a hole injection layer, an electron transport layer or an electron injection layer is called an organic light emitting medium layer.) For extracting the light outside concurrently with applying the voltage to the organic light emitting layer, a construction including a first electrode and a second electrode placed on both sides of the organic light emitting medium layer is provided. This device is constructed by sequentially laminating the first electrode, the organic light emitting medium layer and the second electrode on a transparent board. In general, the first electrode formed on the board is used as an anode, while the second electrode formed on the organic light emitting medium layer is used as a cathode.
An exemplary organic light emitting medium layer may include copper phthalocyanine used as a hole injection layer, N,N′-di(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine used as a hole transport layer, and tris(8-quinolinol) aluminum used as a luminescent layer, respectively. Furthermore, in an attempt to increase light emitting efficiency, the construction of an organic EL element is often made by providing the hole transport layer and the hole injection layer between the anode and the organic light emitting layer, or the electron transport layer and the electron injection layer between the organic light emitting layer and the cathode, which may be selected ad libitum. Any of the substances which constitute the organic light emitting medium layer and allow the layer to function (light emitting medium material) is a low molecular compound, each layer having a thickness of approximately 1 to 100 nm, and deposited by a vacuum evaporation method such as resistance heating.
To the contrary, there are polymer organic EL elements in which a polymer material is used as the organic light emitting medium layer. In the light emitting layer, a low molecular light-emitting material dissolved in a polymer such as polystyrene, polymethyl methacrylate or polyvinyl carbazole, as well as a polymer light emitter such as a polyphenylenevinylene derivative (PPV), a polyalkylfluorene derivative (PAF) or the like may be used. These polymer materials can be subjected to film formation by a wet method such as a coating method or a printing method through dissolving or dispersing in a solvent. Therefore, it is advantageous in that film production is enabled under an ambient air pressure with low equipment costs as compared to the organic EL element in which a low molecular material is used.
In the polymer organic EL element, a hole transport layer is generally provided for the purpose of lowering the voltage to be applied. In typical examples, film formation is carried out using a polymer material ink of a donating molecule and an accepting molecule aggregate which were dispersed in water. It is known that the hole transport layer shows superior charge injection properties. However, in a hole transport layer comprised of a polymer material, since the resistance of the film is high, the film is highly loaded in a high voltage region. Thereby the material itself deteriorates. Therefore, there is a problem in that luminance and electric current density are limited. In this way, the element using the hole transport layer comprised of a polymer material has a resistance problem and there are concern points such as deterioration of light emitting characteristics or reduced life-time.
In addition, it has been proposed that inorganic products such as an oxide, a nitride and an oxynitride of a transition metal, and a p-type compound semiconductor are used for a hole transport layer. In this case, more stable characteristics are obtained than in the case using a polymer material especially in the case of a high voltage and a high luminance region. However, an electron blocking layer comprised of a polymer material between a hole injection layer and a light emitting layer is necessary in order to achieve a high efficiency. (Patent Document 1)
Here, layers such as the above-mentioned hole transport layer, the hole injection layer and the electron blocking layer, the layers being between an electrode and an organic light emitting layer and the layer provided for improving characteristics, are called buffer layers.
Further, in general, the thickness of the electron blocking layer is thinner than the thickness of an organic light emitting medium layer so that an increase in the applied voltage is prevented. Especially, in the case where a display having pixels sectioned for a pixel is manufactured, there are many problems to be solved in order to realize a uniformity of films both inside and outside a pixel when a polymer material is used.
In view of the above-mentioned problems, the object of the present invention is to provide a buffer layer without ununiformity, and to further provide a polymer organic EL element having a high reliability by using a buffer layer having a good resistance and a high stability.
Hereinafter, a known related art is described.
[Patent Document 1] JP-A-2006-114759